How can fish bait spread disease?

­Something sinister appeared in a w­ate­r sample from Canada's Lake St. Claire in 2003. A strain of viral hemorrhagic septicemia (VHS) showed up for the first time in North American waters. Previously, the fish disease-causing virus was confined to Europe. Ships traveling across the pond probably brought it to the Western hemisphere. Since its discovery, VHS ha­s already infiltrated so many freshwater areas surrounding the Great Lakes (except for Lake Superior) that specialists fear that it will get into the Mississippi River.

VHS is particularly troubling because, unlike most fish-related viruses, it affects a wide variety of species. According to the National Parks Service, VHS has killed 28 different freshwater fish species since 2006; yet, more than 50 species may be susceptible to VHS [source: Minnesota Sea Grant]. The virus spreads most rapidly during the spring when fish spawn. Whenever fish populations live in close proximity to one another or are experiencing stress, VHS transmission rates spike as well. Fish send it around to each other through urine, feces, sexual fluids and consumption of infected fish [source: Whelen].

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Once a fish catches the virus, it can lead rapidly to internal bleeding and hemorrhaging from open sores on its body [source: McKenna]. Mortality rates for infected fish fluctuate between 20 and 80 percent [source: Minnesota Sea Grant]. The virus has stricken the round goby fish population the hardest, but keystone fish species, such as bass, walleye and muskies, have also taken a hit.

Here's where the fishing bait issue arises. The states surrounding the Great Lakes have clamped down on fishing bait sales in an attempt to prevent VHS from expanding its reach. Fish contaminated with VHS that are used as bait could pass along the disease in new places. Even dead or frozen fish can contain the active virus. For that reason, bait dealers in the Great Lakes area must certify that the fish they use as bait are disease-free. If they aren't, fishermen can use that tainted bait only in freshwater areas that are known to be infected with VHS. In addition, bait sellers must provide fishermen with a receipt of where the fish bait came from.

Although the regulations are seen as a nuisance by some bait shops and fishermen, they don't restrict fishing activities. VHS kills fish, but it isn't a known human pathogen. That means that fishermen can still catch and eat VHS-infected fish without worrying that they'll get sick. However, if negligent fishermen contribute to VHS' expansion, there won't be many freshwater game fish left to catch in the coming years.